I. Field of the Invention: This invention relates generally to an electrosurgical instrument, and more particularly to an improved blade construction for effecting cutting and coagulation during an electrosurgical procedure.
II. Discussion of the Prior Art: In carrying out electrosurgical procedures, a special scalpel is used which is adapted to be energized by a radio frequency voltage source for cutting tissue and/or coagulating blood through cauterization. Such scalpels commonly incorporate a conductive blade and the RF energy source is connected between that blade and a large area patient plate which is made to abut the skin of the patient at a site generally remote from the location of the surgery. In that arrangement, the system is said to be a "monopolar system". Problems have arisen in using such monopolar systems due to the frequency with which patients suffer burns at the site of the patient plate.
In an attempt to obviate such problems, electrosurgical scalpels have been designed utilizing bipolar blades where first and second conductive electrodes are placed along a sharpened cutting edge on opposite sides of the blade and when a high frequency, high voltage RF energy source is connected across the strip electrodes and brought into contact with tissue, an electrical arc is established which rapidly dehydrates the tissue cells causing them to burst and creating an incision as the blade is drawn across the tissue.
In the Doss et al U.S. Pat. No. 4,161,950, there is described an electrosurgical knife in which the blade is formed using a ceramic substrate which is sharpened to a knife edge and deposited on opposed sides of that substrate and extending close to but short of the apex of the cutting edge are conductive electrodes, preferably formed from tungsten applied using a screening process. The Hren et. al. Pat. No. 4,202,337 also recommends the use of a ceramic substrate for an electrosurgical blade with aluminum oxide (Al.sub.2 O.sub.3) being recommended.
We have found, however, that many ceramic materials including Al.sub.2 O.sub.3 do not possess optimum characteristics for use in the fabrication of electrosurgical blades. When it is considered that the arc produced temperatures may often exceed 1000.degree. C. at the local level, it is important that the volume resistivity versus temperature characteristics be high enough at the arc temperature so that the blade substrate material is not destroyed in our avalanche mode of operation. That is to say, if the substrate material used for the electrosurgical blade exhibits a significant drop in volume resistivity with increasing temperature, a point may be reached where the resistance of the substrate drops to such a low value that the I.sup.2 R loss at radio frequencies increases the blade's temperature even further to the point where an avalanche condition exists, destroying the substrate material and/or the electrode traces formed thereon. Because of this avalanche effect, prior art electrosurgical blades have relied on a sharpened edge to perform mechanical cutting and the RF heating is employed strictly for coagulation. Power levels are limited to a range which does not produce arcing since the resulting high temperatures are destructive of the substrate material.
Furthermore, alumina oxide ceramics tend to be quite brittle and are subject to undue breakage when formed into thin, blade-like substrates. Even modest pressures encountered during electrosurgical procedures have been found sufficient to snap such prior art blades.